feat: add TransBorrow.get_borrowed_quantity method

Co-authored-by: aider (openai/DeepSeek-V3.2-Thinking) <aider@aider.chat>

inventory-backend/app/models/transaction.py

@@ -1,5 +1,6 @@
 from app.extensions import db
 from datetime import datetime
+from sqlalchemy import func
 
 
 class TransBorrow(db.Model):
@@ -46,6 +47,19 @@ class TransBorrow(db.Model):
             'remark': self.remark,
         }
 
+    @classmethod
+    def get_borrowed_quantity(cls, source_table, stock_id):
+        """
+        获取指定库存记录（source_table 和 stock_id）的借出未还数量总和。
+        返回浮点数，若无借出记录则返回 0.0。
+        """
+        result = db.session.query(func.sum(cls.quantity)).filter(
+            cls.source_table == source_table,
+            cls.stock_id == stock_id,
+            cls.is_returned == False
+        ).scalar()
+        return float(result) if result is not None else 0.0
+
 
 class TransRepair(db.Model):
     __tablename__ = 'trans_repair'